The invention relates generally to phased antenna arrays and more particularly to side-lobe suppression systems as applied to phased antenna arrays.
Antenna arrays consist of an arrangement of closely spaced antenna elements uniformly spread over the antenna area or aperture. The beam pattern for phased array antennas includes a principal or main lobe used for detecting a target and several side-lobes of reduced radiation energy grouped around the principal lobe. The direction and shape of the beam is controlled by altering the phase and amplitude of the individual elements. A problem with airborne antenna systems is that side-lobes directed towards the ground pick up ground clutter, which can interfere with echoes detected by the principal lobe and severely reduce the radar""s ability to detect weak target echoes. Conventionally, side-lobe clutter is mitigated by reducing the side-lobe radiation power by amplitude weighting of the aperture. Amplitude weighting works well on reception, introducing only a limited and acceptable power loss. However, in a system employing an active phased array, amplitude weighting on transmission involves an unacceptably high power loss, typically of around 5 to 6 dB.
An alternative technique to amplitude weighting is described in U.S. Pat. No. 4 939,523. This involves weighting or tapering the phase of the antenna elements only. The resulting beam has a sharply focussed main lobe and an asymmetric arrangement of side-lobes, with the side-lobes in the lower hemisphere of the antenna, that is the side-lobes directed towards the ground, being low. However, the implementation suggested in this reference is very complex. The phase distribution across the antenna aperture is obtained using an inverse Fourier transform, and the phase variation across the aperture must be recalculated, or table-interpolated, during the roll of the aircraft to maintain low side-lobes towards ground.
There is thus a need for an antenna implementation whic minimises side-lobe clutter with acceptable power-loss on transmission, which also performs well on reception and which is simple to implement.
According to the invention, an antenna system is proposed having an array of antenna elements and phase control circuitry for setting the phase of signals that may be fed to and received by each element. The phase control circuitry includes a phase correction arrangement for correcting the phase of each element as a function of the position of the element within the array.
Preferably the phase correction is proportional to a first function of the angular position of the element and proportional to a second function of the radial position of the element relative to a central point in the array. The first function is a sinusoid of angular position of the element and the second function is an odd polynomial of the radial position of the element having at least one term of third order or above. The phase correction arrangement may comprise several modules, wherein each module is associated with a single or more than one element. The invention also relates to a method for suppressing side-lobes.
By providing the phase correction as a simple function of the position of each element expressed in radial and angular position, the implementation is rendered very simple; in particular, the phase correction may be calculated element by element. Furthermore, changes in the orientation of the antenna, such as during aircraft roll, for example, may be compensated for simply by a shift in the origin of the angular term of the function for each element in the array. The resulting beam pattern includes low side-lobes in the lower hemisphere and gives rise to only limited and acceptable power loss on transmission.